Rathman



G. RATHMAN June 2o, 195o Scan Puur l 2 Sheets-Shut 2 Filed N0v. 2. 1946JNVENmn. 645597" @Q7/,WAN

Patented June 20, 1950 SCREW PUMP Gilbert Rathman, Union, N. J assignorto Roots- Connersville Blower Corporation, Connersville, Ind., acorporation of Indiana Application November 2, 1946, Serial No. 7077.38414 claims. (c1. 23o-143) The invention relates to a screw pump of thetype which includes two helically ribbed rotors, one usually called adischarge rotor and the other called a mating rotor; and which rotorsare in intermeshed engagement and organized for passing, or for beingpassed by, a compressible or expansible fluid, specifically air or othergas. The invention is a development of a similar form of pump disclosedin my co-pending application entitled Screw pump, Serial No. 511,083,filed November 20, 1943 (now Patent No. 2,460,310 dated February 1,1949).

In this, as well as in the device of my earlier application, the primaryobject is to provide a highly efiicient type of screw pump capable ofmaintaining a constant flow of the gas or other medium passedtherethrough without objectionable surging or other variable pressureconditions in the fluid at the discharge end of the pump.

The primary object of the present disclosure is still further to improvethe efficiency of screw pumps of the type above identied; to retain theadvantages above recited and at the same time to simplify and thusreduce the manufacturing cost of such pumps specifically in the form ofvalvular outlet control disclosed in the earlier application.

In both the previous disclosure and in this disclosure, the fluid at thehigh pressure end of the mating rotor is blocked off from direct escapefrom the system and must flow into the discharge rotor and hence outfrom the system through the high pressure end of the discharge rotor. Inthe co-pending application the discharge is controlled by a valvulardevice composed of two apertured coacting valve elements, one fixed tothe casing and the other to the discharge rotor and turning therewith toopen the high pressure end of the discharge rotor once for eachrotation. This method of utilizing a distinctive valve in controllingthe pump discharge is sometimes called the valve method,

In the device herein disclosed that valve elementVA which in the earlierdevice rotates with the discharge rotor has been discarded and thepressure developed at the high pressure end of the discharge rotor iscontrolled by a valvular mechanism which does not include a separate anddistinct rotative element and is controlled solely by reason of thepreset configuration and volumetric discharging capacity of a fixeddischarge port which is opened and closed periodically by the rotationof the discharge rotor alone. In distinction from the valve method ofthe prior application, the present disclosure for setting the dischargepressure is called a port method.

Briefly, the present disclosure distinguishes from the prior art in thatthe intermeshing threads of both rotors, or the screw elements of bothrotors, are convex on one side, concave on their opposite sides; thethreads inter-mesh with a concave side of one thread facing the concaveside of the other thread, and particularly distinguishes from the priorart in that the discharge opening faces the concave side or face of thethread of the discharging rotor.

Various other objects and advantages of the invention will be in partobvious from an inspection of the accompanying drawings and in part willbe more fully set forth in the following particular description of oneform of mechanism embodying the invention, and the invention alsoconsists in certain new and novel modifications of the preferredconstruction and combination of parts hereinafter set forth and claimed.

In the accompanying drawings:

Fig. l is a plan view of a preferred embodiment of the invention withthe casing shown in longitudinal section and taken on the line I-I ofFig. 2;

Fig. 2 is a view in side elevation of the device shown in Fig. 1 withthe casing shown in longitudinal section and taken on the planeindicated by the line 2-2 of Fig. 1;

Fig. 3 is a view in cross section taken on the line 3-3 of Fig, 2,looking inwardly towards the discharging rotor and showing a fixeddischarge opening of maximum discharging capacity and thus of leastpressure transmitting capacity;

Fig. 4 is a similar View in cross section, also taken on the line 3-3 ofFig. 2, but showing a fixed discharge opening of minimum dischargingcapacity and thus of maximum pressure transmitting capacity, and

Fig. 5 is a view in cross section taken anywhere along the length of theintermeshing screws and in the instant case taken on the line 5-5 ofFig. 2.

In the drawing there is disclosed a pump casing IIl wide open atopposite ends and provided on one side with a fluid intake port I I andon the opposite side with a fluid discharge port I2. Considered in crosssection as shown in Figs. 3-5, the casing is a, casting of an 8-shapedform and is composed in effect of two hollow cylinders I3 and I4 inpartially overlapped relation and with their axes in parallel and offsetrelation. The open ends of the casing are closed by a pair of hollowheads I5 and IB demountably secured to asuma the flanged ends of thecasing by bolts I1. Journalled in sets of combination radial and endthrust bearings |8 contained in the heads I5 and |6 are a pair of rotorsfor forcing gases and other fluids from the intake port through thecasing and out through the discharge port I2.

One of the rotors |9, hereinafter referred to as the driving rotor,includes a shaft 20 which projects at one end exteriorly of the casingand of the head I6 and is adapted to be connected to a source of powerarranged to turn the rotor as indicated by the arrow at the right ofFig. 1. The other rotor 2|, hereinafter referred to as the driven rotor,includes a shaft 22 which is connected to the shaft of the driving rotorby means of a meshed gear set 23 contained in the head I6 and organizedto drive the rotors at a one-to-one ratio.

The driving rotor I9 includes a discharge screw 24 keyed to the shaft 20and held against movement towards the right of Figs. 1 and 2 by a stopshoulder 25 formed on the shaft. The screw 24 is formed with a singlethread 26 of at least one convolution and has a running fit in thecylinder I3. Similarly the driven rotor 2| is provided with a matingscrew 21 keyed to the shaft 22 and likewise held against a stop shoulder28 fixed to the shaft 22. The screw is formed with a single thread 29,is of the same length as the driving screw 24 and has a. running t inthe cylinder I4.

The screws are in intermeshing relation and in general the device asthus far described is of somewhat conventional construction.

Fitted into the left or high pressure and discharge end of the cylinders|3 and I4 are two hollow castings 30 and 3| through which loosely extendrespectively the shafts 20 and 22. The screws are thus fitted axiallybetween the castings 30 and 3| and the shoulder stops 25 and 28 on theshafts. It is noted that as the screws are not confined closely betweenheads of the casing there is no necessity to provide clearances foraxial thermal expansion and contraction as is necessary in similar knownforms of such pumps. These castings Sil-3| are held between the screws24 and 21 and the head I5. Casting 30 may be replaced by a similarcasting with different types of discharge openings as hereinafterdescribed, simply by removing the head l and substituting the properport block 30. These castings coact to provide an annular channel 32 atall times open to the discharge port I2 as shown in Fig. 1. The innerwall 33 formed by the inner aligned sides of the two castings when solocated in position provide a barrier for the entire left or dischargeend of the mating screw 21 and at least for a portion of the left ordischarge end of the discharge screw 24. The barrier wall 33 in theportion thereof formed by the block 30 is provided with a singledischarge opening 34 which is located within the outlines of the axiallyprojected cross section of the discharge screw 24 and leads therefrominto the channel 32 and therefrom to the outlet port I2 as shown inFigs. 1 and 2.

The arc length of the opening 34 and thus its discharging cross sectionand its relation to the end of the opening or channel in the dischargingscrew 24, controls the degree of compression which the pump is intendedto impose upon the fluid passing therethrough. If it is the intent touse the pump simply as a liquid, air or gas propeller or transport andwithout intent to create any internal or built-in compression on thefluid, the opening is of the maximum extent as shown in Fig. 3. Thismaximum port opening begint at point a Where the cylinder boresintersect and extends circumferentially and clockwise for about 220degrees to the point b. From point b to point c the port outlineterminates in a'short concaved curve. The hub diameter circlehereinafter referred to as the root circle extends for about 320 degreesfrom point d to point c. The other end of the port outline terminates ina convex curve from point d to point a and coincides with the outsidecircumference of the mating screw 21. This means that the maximum arclength of the discharge opening 34 measured clockwise from a to b isabout 220 degrees.

Where it is desired to utilize the device as a pressure creating pumpthe angular length of the opening 34 is reduced from the maximumsuggested in Fig. 3 in the direction of rotation of the discharge screw24. Fig. 4 illustrates one such high pressure discharge opening 35 andis formed in effect by advancing the curved edge b-c as shown in Fig. 3.counter-clockwise to the position a--c in,Fig. 4 forming a curvilineartriangle, and at about which point the maximum eiiicient workingpressure ratio is obtained. In any case the area of the opening 35 isless than the discharging area of the groove 40, so that as the groovepasses the opening the fluid under whatever pressures are developed inthe discharging end of the groove, are released at that point in threadrotation where the groove 40 and outlet 35 register. Instead of blockingoff the maximum opening 34 of Fig. 3, to a maximum extent as shown inFig. 4, lesser arc lengths may be provided for the opening 35 asdesired. The dotted lines marked e and f in Fig. 4 indicate the rightends of two such less openings. It is understood that graduallyincreasing the area 35 from that shown in Fig. 4 towards that shown inFig. 3 gradually reduces the pressures imposed on the fluid passed bythe pump. In practice that port block or casting 30 is used whichcontains the opening having an arc length, between that shown foropening 34 and that shown for opening 35, which will give thecompression desired for the fluid as it is discharged from the pump.

It is particularly noted that one side of the thread or rib of eachscrew is concave in cross section and the opposite side is convex. Forinstance, and referring first to the thread 26 of the driving screw I9,it is seen the left side 36 as viewed in Figs. 1 and 2 is concave whilethe opposite or right side 31 is convex, or almost so. Referring to thethread 29 of the mating screw 21, the left side 38 is convex, orsubstantially so,

while its opposite side 39 is concave. The screws are arranged so thatwhen interme'shed as shown in Fig. 2 the concave side of the thread ofone of the screws faces the concave side of the thread of the otherscrew. Likewise, of course, the convex side of one of the screws facesthe convex side of its mating screw. For instance, the concave rightside 39 of mating screw thread 29 faces the concave left side 36 of thedischarge screw thread as clearly shown at the center of Fig. 2.

It is a particular feature of novelty of this disclosure that theconcaved side 36 of the thread 26 forming the discharge screw faces thedischarge opening 34 or 35 as the case may be. 'I'his means that theconvex side 38 of the mating screw 21 faces the end barrier formed bythe solid wall 33. It is a further refinement of the instant disclosurethat the thread 26 of the discharge screw 24 is relatively thin in axialcross section of material with a relatively wide groove 40 between itsturns. The thread 2! of the mating screw 21 is relatively thick in axialcross section of material with a relatively narrow groove 4I between itsturns.

By using this arrangement the discharge rotor Y absorbs a largerpercentage of power supplied to the other and reduces the dynamicunbalance.

which exists in intermeshing rotating screws of the type hereindisclosed.

The interior of the cylinders I3 and I4 in the length dimension thereof,between the inner wall 33 and the head I6, may be considered asproviding screw containing spaces in which the screws freely rotate.Between the right ends of the screws and the head I6 is provided anintake chamber 42 of relatively large fluid transmitting capacity, opendirectly from the intake port II and forming the suction or intake endof the system and wide open to the intake ends of the two screws.

It is the intent herein to design the two rotors so that when installed'in the pump they will be in static and dynamic balance. This isattained first by carefully machining the screws to approach as closelyas possible this ideal condition, and thereafter each rotor is modifiedto obtain such balance either by adding or removing material from thescrews following conventional practices in balancing high speed rotatingbodies. It is f course possible to core out material from the mass ofmetal forming the threads and under some circumstances this ispreferable as it provides screws less bulky than herein illustrated. Inone instance metal was carefully removed from one side of each screw bya longitudinal grooving of the outer perimeter of each thread.

The rotors both have an outer peripheral surface of cylindricalcurvature as shown at 43 to conform respectively to the curvature of thewall of the casing in which it rotates. The perimeter of the dischargescrew 24 within the outlines of its associated thread 26, and the screw21 within the outlines of its associated screw 29, each defines aroot-circle or dedendum circle 44 concentric with its associated shaft.Each rotor exhibits in any section perpendicular to its axis, as shownin Fig. 5, a compound Acurved outline whose ends meet the peripheralcircle 43. One outer portion of each of the compound curved outlines onone side of its associated root circle is concaved as shown at 45 andthe other outer portion on the other side of its root circle is convexas shown at 46, with the part of the compound curve at its centerconnecting the concaved and convex portion forming the exposed part ofthe root-circle 44.

The convex portion 46 between the root-circle time being and that thedischarge screw 24 is turning counter-,clockwise and the mating screw2'I is turning clockwise as indicated by the arrows in Fig. 5. then thefluids are picked up from the large volume intake chamber 42 at thesuction end of the device facing the free ends o! the screws, and ismoved thereby axially from right to left of Figs. l and 2 towards thedischarge port 24. The part of the fluid which is picked up by themating screw 2l is transmitted along the passage-way formed by theconvex sides of the meshing screw threads and the wall of the cylinderI4 and then transversely to the discharge rotor which in turn dischargesit from the pump through the port 34, channel 32 and port I2. There isthus provided a positive displacement type pump or blower of thepull-back type giving a square card performance.

Now assuming that a reduced area opening such as 35 of Fig. 4 is usedand under the same conditions of screw rotation, the uid transmittedthrough the pump is brought from zero compression in the chamber 42 anddelivered under such pressure as may be determined by the arc length andlocation of the opening 35 through which the fluid. compressed by thescrews, is discharged. In this case there is produced an internal orbuilt-in compression in the last turn of the combined discharging, andin this case compressing, screw 24 resulting in a compression type cardand giving higher efliciency than has been known heretofore in pumps ofthis character. The discharge is smooth, without shock or wire drawing.

While only one pair of intermeshing screws are shown, it is obviouslywithin the scope of the disclosure, as taught by the prior art toduplicate the arrangement illustrated in Fig. 1 on opposite sides of thedischarge conduit 32 as in the co-pending application, and it ispossible to use two .or three discharge screws of the type hereinfeatured grouped about and meshing with one centrally disclosed matingscrew. It is also possible as taught by the prior art to utilize thepump as a motor by admitting actuating expansive fluid into the pumpthrough the normal discharge port I2 and discharging the fluid after ithas expanded out from the pump through the normal intake port Il.

I claim:

1. A screw pump including a casing provided with an intake port and witha discharge port, a pair 0f rotors mounted on the pump for rotarymovement, means connecting the rotors to drive one from the other at aone-to-one ratio, each of said rotors provided within the casing with ascrew formed of a spiral thread, said screws being in intermeshingengagement at all times and power driven to advance fluids through thecasing from the intake port through the discharge port, the spiralthreads being each concave on one side and convex, or substantially so,on its opposite side,` said screws related to cause the concave side ofone thread to face the concave side of the other thread, and with theconvex side of one thread facing the convex side of the other thread,the intake ends of both screws being wide open to receive fluids fromthe intake port, means forming a wall at the opposite or discharge endsof both screws and against which the screws squarely abut, said wallprovided with a discharge opening leading to the discharge port andotherwise closed, said opening having an arc length not materiallygreater than 220 degrees, being within the axially projected outline ofone of the screws and facing the concave side of the thread of the last-mentioned screw the groove of last mentioned screw in the end facingthe wall having a discharge opening with an arc length in about 200,said wall forming a, barrier at the discharge end of the other screw toprevent flow from said other screw directly into the discharge port.

2. In a screw pump of the port type, the combination of a casingproviding a screw containing compartment one end of which forms a fluidintake chamber, a pair of shafts extending in parallel relation throughsaid compartment, means for driving one shaft from the other, each shaftprovided at the inner side of the intake chamber with a stop shoulder, apair of intermeshing screws one secured to one of the shafts and theother secured to the other shaft and abutting their associated stopshoulders, a wall extending across the compartment at the discharge endsof the screws, opposite the ends abutting the stop shoulders and againstwhich the discharge ends of the said screws abut, said wall providedwith a single opening located within the axially projected outline ofone of the screws, having an arc length materially less than 220 degreeswhereby the opening is periodically opened and closed by the screw whichfaces it, once for each rotation of said screw and said wall actingdas abarrier at all times closing the discharge end of the other screw.

3. `A screw pump including a casing provided with an intake port and adischarge port', a pair of rotors carried by the casing and mounted forrotary movement, means connecting the rotors to drive one from theother, each of said rotors provided within the casing with a screwincluding a single spiral thread having at least one convolution, thethreads of said screws being in intermeshing engagement and operative toadvance fluids through the casing from the intake port t the dischargeport and to place the same under pressure, the spiral threads being eachconcave on one side and convex on the other side, said screws disposedwith the concave side of one of the threads facing the concave side ofthe other thread, the intake ends 0f both screws being exposed to theintake port to receive the fluid therefrom, means forming a wall at theopposite or discharge ends of the screws, said wall provided with adischarge opening with an arc length less than the arc length of theconcaved end of the screw which faces it leading to the discharge port,with said opening facing the discharge end of one of the screws andfacing the concave side of the thread of said facing screw, and saidwall otherwise closing the discharge ends of both screws.

4. In a device of the class described, the combination of a structureproviding a screw containing chamber, means for propelling a fluidthrough the chamber and for imposing a pressure on the fiuid as it ispassed therethrough, said means including a screw having a spiral threadwith a running fit in the chamber, one side of said thread beingconcaved, and a wall defining one end of said chamber and facing theconcave side of the thread, the sole outlet from said chamber consistingof an opening in said wall whose cross-sectional area is less than thecross-sectional area of the groove between the turns of the thread atthe discharge end of the screw, said opening thus controlling by reasonof' its cross-sectional area the degree of compression imposed on thefiuid by said means.

5. In a device of the class described, the conibination of a structureproviding a screw containing chamber, means for propelling a iiuidthrough the chamber. said means including a screw having a spiral threadwith a running fit in the chamber, one side of said thread beingconcaved, a wall facing the concave side of the thread, defining one endof the chamber and against which the screw abuts, said wall providedwithin the axially projected outline of said thread with a dischargeopening, the groove between the turns of the thread at the end thereofabutting the wall coacting with the opening to register therewith oncefor each rotation of the screw and to thus provide a valvular device forcontrolling the discharge of the fluid from the chamber, the arc lengthof the end of the groove facing the discharge opening being greater thanthe arc length of the opening. 6. In a screw pump, the combination of apump casing provided with an inlet port. a discharge port and a screwchamber, means including a pair of intermeshing screws mounted in thescrew chamber for propelling a fluid from the inlet port, through thescrew chamber and out through the discharge port, means for driving onescrew from the other at a one-to-one ratio, a wall defining the end ofthe screw chamber at the discharge ends of the screws, said wallprovided within the axially projected outlines of one of the screws withan opening adapted to coact with the adjacent end of the screw facingthe same to provide a valvular means for opening communication betweenthe screw chamber and the discharge port once for each rotation of thescrews, said opening being arc shape with its inner and outer edgesconcentric and each extending ,for not more than 220, with one enddefined by a short concaved curve and its other end by a short convexcurve connecting said inner and outer edges and in which the thread ofthe screw which faces said opening is concaved on the side facing theopening.

7. In a screw pump, the combination of a pump casing, mechanism forpropelling a fluid through the casing and for imposing a pressurethereon, said mechanism including a pair of rotors, each including ashaft and provided with a spiral thread, with the threads intermeshing,means for driving one rotor from the other, each thread being concave onone side and related with the concave side. of one thread facing theconcave side of the other thread, a barrier at the discharge end of thethreads through which the two shafts loosely extend, and against whichboth threads abut, said barrier provided within the axially projectedoutline of that thread whose concave side faces the barrier with adischarge port, and said port outlined by at least three sides. one ofwhich is convex and coincides with the axially projected outline of theperimeter of the other thread, another side of which is concave andextends linwardly from the casing towards the shaft which forms part ofthe discharge thread and whose third side is a circular arc concentricwith said last named shaft.

8. The screw pump defined in claim '7 and in which the discharge port isof less area than the groove of the thread which discharges into thesame, once for each rotation of the thread.

9. In a device of the class described, the combination of a pump casing.mechanism for moving a fluid through the casing and including a pair ofrotors journalled in the casing and each provided with a spiral threadwith the threads in intermeshing relation, means for driving one rotorfrom the other, the thread of each rotor being concaved on one side andwith the concave side of one thread facing the concave side of the otherthread, means forming a barrier at the discharge end of the threads andagainst which the threads abut. said barrier in the prtion thereof whichfaces the concave side of one of the screws provided with a dischargeopening and said barrier being otherwise closed.

10. In a device of the class described, the combinationof a pair ofintermeshing helical rotors mounted to rotate about parallel axes andintergeared to drive one from the other, each rotor having an outerperipheral surface of cylindrical curvature, each rotor exhibiting inany section perpendicular to its axis a compound curved outline whoseends meet the outer cylindrical curvature with one outer portion of thecurved outline concaved and the other outer portion convex and with thepart connecting the concaved and convex portions forming an exposed partof a root-circle concentric with the axis of rotation of the associatedrotor, and the convex portion between the root-circle and thecylindrical curvature being a plane, simple curve substantially that ofa portion of an Archimedean spiral. a casing whose inner wall conformsto the paths of the outer cylindrical surfaces of said rotors, and powermeans for driving the rotors.

11. The device defined in claim 10 and wherein a wall closes the casingat one end of the rotors and against which wall both rotors abut, thewall in the portion thereof facing one of the rotors provided with adischarge opening located within the outlines of the axially projectedcross section of said one rotor and located exteriorly of its projectedroot-circle, one edge of which opening coincides in one position of saidone rotor which said convex portion of said one rotor, and said onerotor coacting with the opening to form a valve for regulating thedischarge from the casing once for each rotation of said rotors.

12. In a positive displacement rotary type compressor, the combinationof a pump casing open at opposite ends and provided at one end with afluid intake port, a rst closure for the casing at the end provided withsaid port, a second closure for the casing at its opposite end andprovided with a discharge opening, said second closure being readilydemountable from its end of the casing and fashioned to be replaced by asimilar closure having a discharge opening of a different size, a pairof intermeshing screws iltted for free rotary movement in the casing andinterconnected to drive one from the other, said screws having theirends journaled in the closures, each screw bearing at one end axially onsaid second closure and at its other end disposed in spaced relation tothe first closure to form a space at the intake ends of the screws wideopen to the fluid intake port, one of said screws constituting adischarge screw with the channel between its threads at its dischargeend facing the discharge opening, said discharge opening having an arclength less than the arc length of the channel formed between the threadturns of the discharge screw and thus less than the discharge area ofthe discharge screw.

i3, In a device oi' the class described. the combination of a pumpcasing, mechanism including a pair of intermeshing screws mounted forfree rotary movement in the casing to advance a fluid axially throughthe casing, one of said screws forming a fluid discharge screw and oneside of the thread of which discharge screw is convex in cross sectionthrough the screw, a closure for the casing at the discharge ends of thescrews and against which both screws abut, said closure provided with asingle discharge opening facing the. adjacent end of the discharge screwand oiset from its axis, and said closure otherwise closing the end ofthe casing at the discharge end of the screws, said opening being in theform of a curvilinear triangle whose area is materially less than theopening between the threads of the discharge screw at its discharge endand one side of which opening has a concaved curve substantially thesame as the convex curve of the thread.

14. In a device of the class described, the combination of a pair ofintermeshing helical screws of the same external diameter andpitchcircle, mounted to rotate about parallel axes and intergeared to driveone from the other at a oneto-one ratio, each screw having an outersurface of cylindrical curvature defining its pitchcircle, each screwexhibiting in any section perpendicular to its axis a compound curve,with one outer portion concaved and the other outer portion convex andwith the portions connecting the convex and concave portions forming a'hub whose perimeter forms a root-circle concentric with the axis ofrotation of the associated screw, a wall against which both rotors abut,the wall provided within the axially projected area of one of the screwswith a discharge outlet coacting with said screw to form a valve openonce for each rotation of the screws, said outlet exhibiting in a.similar cross section perpendicular to the axis of said screw an arcuateform and whose inner outlining edge is concaved and coincides with aportion of the rootcircle of said screw, and the outline of whichdischarge outlet is also dened by two curves caved and coinciding with aportion of the pitch circle of the other screw and the other of said twocurves being convex and coinciding with the convex side of theassociated screw, and a casing whose inner wall conforms to the paths ofthe outer cylindrical surfaces of the screws.

GILBERT RATHMAN.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 165,805 Disston July 20, 1875960,992 Motsnger June 7, 1910 960,994 Motsinger June 7, 1910 1,233,599Nuebling July 17, 1917 1,306,169 Brooks June 10, 1919 1,751,703 LongMar. 25, 1930 2,095,168 Burghauser Oct. 5, 1937 2,188,702 BurghauserJan. 30, 1940 2,287,716 Whitfield June 23, 1942- 2,460,310 Rathman Feb.1, 1949

